drm/panel: simple: use drm_panel backlight support

Use drm_panel infrastructure for backlight.
Replace direct calls with drm_panel_*() calls
to utilize the drm_panel support.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191207140353.23967-4-sam@ravnborg.org